1. Technical Field
The present invention relates to a driving apparatus and a semiconductor apparatus for driving an alternating-current motor of a home electric appliance or an industrial facility.
2. Background Art
In recent years, in the above-mentioned product field, methods for driving brushless motors by inverters have been expanding. In the field of home electric appliances, the price competition has intensified, and thus there is a demand for inexpensive inverter units. Therefore, for an inverter-driving apparatus with a brushless motor, a 120-degree energizing method which permits a simple circuit configuration and relatively high motor-efficiency is used.
In a motor driving circuit using the 120-degree energizing method, a current is applied to two of three-phase windings for driving the motor by detecting a magnetic-pole position of a motor rotor by a magnetic-pole position detector and controlling the on/off state of each switching element of an inverter unit in response to the time at which the magnetic flux of the rotor is orthogonal to the magnetic pole of a stator.
In this 120-degree energizing method, it is known that torque ripples take place at times of switching the windings to be energized (commutation), and thereby noise is generated.
Further, if a motor current waveform includes a large amount of higher harmonic component, motor torque generally tends to have ripples. The motor torque is a product of motor-specific inductive voltage and motor current, and thus largely depends on the motor current waveform. The torque ripples vibrate the motor itself, which causes a base where the motor is mounted to vibrate. This vibration results in noise.
As a method for reducing the noise, there is a method in which a motor driving current is made sinusoidal by means of so-called PWM (Pulse Width Modulation) control. Specifically, a sinusoidal modulated wave signal, which is synchronized with the position of the rotor, is generated, a PWM signal is generated by comparing the modulated wave signal with a carrier wave signal, and a sinusoidal wave PWM voltage is outputted from the inverter. Then, in order to generate the sinusoidal modulated wave signal synchronized with the position of this rotor, a micro computer is used as well as a PWM timer embedded therein. However, this method requires a complicated and expensive apparatus in comparison with the 120-degree energizing method.
In contrast, as a method for achieving a motor driving circuit with low noise using a relatively simple circuit, there is a method for reducing motor noise by smoothening current waveforms, wherein an analog circuit is used for generating a sinusoidal wave voltage instruction and three levels of voltage signal waveforms are smoothed using a filter circuit (refer to a patent document 1, for example).
Additionally, there is a method of generating a quasi sinusoidal wave with less distortion (refer to a patent document 2, for example).
Reference Document 1: JP Patent Publication (Unexamined Application) No. 2001-251886
Reference Document 2: JP Patent Publication (Unexamined Application) No. 11-341863
According to the method disclosed in JP Patent Publication (Unexamined Application) No. 2001-251886, there remains large distortion in modulated waves, and therefore torque ripples virtually remain. In comparison with the 120-degree energizing method, it does not produce an effect large enough to substantially reduce noise when driving a motor.
On the other hand, according to the method disclosed in JP Patent Publication (Unexamined Application) No. 11-341863, the size of the circuit is considered large, and this is not convenient for integrating the circuit into a monolithic IC.
The present invention has been devised by taking the above-mentioned points into account, and an object of the present invention is to provide a driving apparatus for an alternating-current motor capable of reducing torque ripples with a relatively simple circuit.
In order to overcome these defects of the conventional technologies, according to the present invention, an inverter is PWM-controlled by generating a plurality of trapezoidal wave signals having at least two constant levels based on a position sensing signal of magnetic pole of a motor, generating a quasi-sinusoidal wave signal at a quasi-sinusoidal wave modulator from the plurality of trapezoidal wave signals, and comparing a carrier wave with the quasi-sinusoidal wave signal.